Control of parallel dc dc converters in a dc microgrid
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This document discusses controlling parallel DC-DC converters in a DC microgrid using a virtual output impedance method. It begins with introductions to power converters and DC converters. It then discusses types of DC converters, their performance parameters, output impedance, and how output impedance mismatch can cause circulating currents. It proposes using a virtual output impedance control loop to compensate for differences in converter output voltages and reduce circulating currents. Simulation results show this method can decrease circulating currents to near zero while maintaining voltage regulation within allowed deviations.
Control of parallel dc dc converters in a dc microgrid
- 1. 1/12 Power Electronic Systems & Chips Lab., NCTU, Taiwan Control of Parallel DC-DC Converters in a DC Microgrid Using Virtual Output Impedance method 電力電子系統與晶片實驗室 Power Electronic Systems & Chips Lab. 交通大學 • 電機控制工程研究所 台灣新竹‧交通大學‧電機控制工程研究所‧電力電子實驗室~鄒應嶼 教授 Sushil Kumar
- 2. 2/12 Outline 1. Introduction 2. Types of DC Converter 3. Performance parameter of DC converter 4. What is output impedance ? 5. Circulating Current and output Impedance 6. Microgrid 7. Control of DC-Dc converters by using virtual impedance 8. Droop control Startagy 9. How Virtual impedance work? 10. Simulation Study 11. Advantages and disadvantages of this scheme 12. Conclusion 13. Question Arises
- 3. 3/12 Introduction Power Converter:- For the control of electric power, power conditioning the conversion of one form to another from is necessary that is done by static power converters DC Converter A DC converter is an electrical circuit which accepts a DC input and generates a DC output of a different voltage. Usually achieved by high frequency switching action employing inductive and capacitive filter elements DC converter is used for unregulated DC to regulated DC. :
- 4. 4/12 Types of DC Converter 1. Buck Converter :- Voltage step down and current step up 2. Boost Converter:- Inverse of Buck, Voltage up ,current down 3. Buck Boost converter:- Output Voltage may be less then or higher than input voltage, as per user requirement, hybrid of Buck and boost both hence buck-boost.
- 5. 5/12 Performance parameters of DC-DC converters There are many performance parameter whose take attention while we design the converter. These are:- 1. Operating Frequency 2. Inductor Selection 3. Capacitor Selection 4. Switch component
- 6. 6/12 What is Output Impedance ? • The output impedance is defined as the output voltage response of a converter for the excitation of current (load current, VL) at constant input voltage (Vin) and a specific value of duty ratio D. • Output impedance is mismatch due to unequal output voltage or variation in current. • It is very necessary that output impedance of output converters should be same.
- 7. 7/12 Circulating Current and Output impedance • The mismatch in the output voltages will increase the current sharing difference and flow of circulating current. • We can also say that output impedance mismatching is responsible for circulating current or vice versa. • The circulating current phenomena we can understand by this table 1. Where, Vdc1, Vdc2 are the output currents R1, R2 are cable resistances, I1 and I2 are o/p currents and Ic12 and Ic21 are the Circulating currents of two converters respectively.
- 8. 8/12 Microgrid Microgrid :- An active power distribution network Capability of autonomous distribution Renewable energy sources connected with common bus Can say Local power station Work as a autonomous as well as tie with Grid Types:- AC Microgrid DC Microgrid
- 9. 9/12 Control of DC-DC Converters by using Virtual Impedance Method Importance of Virtual Impedance :- Virtual impedances can be used to match the grid-side impedance in AC microgrids The Virtual Impedance can be programmed at the certain frequency to form the expected harmonic impedance, which enables the function of harmonic sharing and damping. Virtual Impedances can be employed to realize the resonance damping in the LCL filters. By using the additional current sensors and multiple control loops, the resonance peaks imposed by LCL filters can be attenuated.
- 10. 10/12 Continue… The virtual impedance loop can either be embedded as an additional degree of freedom for active stabilization and disturbance rejection, or be employed as a command reference generator for the converters to provide ancillary services. connect a virtual impedance in parallel or series with the input impedance of the load converter , the magnitude or phase of the load converter’s input impedance is modified in a small range of frequency, to solve the instability problem of a cascaded system
- 11. 11/12 Droop control strategy using virtual impedance Three control levels:- Primary control:- Voltage and Freq. stabilization after islanding Secondary Control:- check for any deviations caused by primary Tertiary control:- Govern the power flow between Microgrid & main Grid Droop Control Method:- It is method to share load between parallel connected sources In order to obtain proper load sharing we use decentralized droop control method i.e. No communication between the sources hence name decentralized.
- 12. 12/12 Droop control in DC Microgrid • In the parallel DC-DC Converter, the output voltage of the converter given a droop with increase in load power. Fig.1. DC Microgrid Architecture Fig.2. P-V Droop Characteristics As P1max < P2max, the sloop or droop of voltage converter 1 is greater than that of 2. At any particular voltage V*, the power shared by Converter 1 is P1* and that of converter 2 is P2*. The total load is sum of P1* and P2*
- 13. 13/12 Effect of droop control • Voltage regulation is adverse effected by droop control so the slope of droop characteristics is designed keeping in mind the max. value of voltage deviation. • Cable resistance that is connected to source and point and common coupling is also mentioned because the higher value of resistance has poor voltage regulation and due to this we can not give more sloop to the droop characteristics. Load sharing will be better if cable resistance is less. Fig.3. Effect of cable resistance on P-V droop Characteristics
- 14. 14/12 Effect of droop control • When droop gain used is higher, current sharing error of the converters reduces • Voltage regulation is increased • High value of droop gains provide stabilizing effect on DC Microgrid but also increase losses. A trade off is required is selecting the droop gain as a small value and a higher value can adversely affect the system
- 15. 15/12 Droop Control using Virtual impedance Two DC-DC converters connected in parallel The equivalent ckt. DC power supplies connected to a common load through resistive output impedance. Nominal output voltage should be same. In case of minor difference of output voltage , a circulating current flows between the sources Virtual Impedance is programmed to reduce the circulating current
- 16. 16/12 How virtual impedance work ? The Control Input to the voltage loop expressed as:- Vo* = Vref – RdIo ……………….(1) Where Io is the measured output current RD is the virtual impedance Vref is the no load output reference voltage If Ev is the maximum output deviation, RD and Vref is Vref = Vn – Ev/2 …………(2) RD = Ev/ Imax ………….(3) Where Vn is the nominal output voltage Imax is the max output current The virtual impedance control loop compensate the difference in voltage reference Vo* = V*o1-V*o2 The current sharing between the two converters are Δ Io = Io1-Io2= Δ Vo* /RD
- 17. 17/12 Simulation Study • Simulation study done by connecting two buck converter back to back. The parameter used are given in table 1. • The design parameter given in Table 2. The design equation of the buck converter L= 1.25 (1-D)RL / 2f C= (1-D) / 8Lf2 (ΔVo/Vo) Where, D is the duty ratio ΔVo is the peak to peak ripple output Voltage, Vo is the reference voltage
- 18. 18/12 Design Parameter Nominal output voltage = 380 V Nominal Output voltage = 120 V Virtual Output impedance = 8 Ω Proposed control strategy output time = 0.5 s Sudden increase in load at 0.8 s Load resistance change from 100 Ω to 80 Ω Maximum Voltage deviation= 5 % Current before applied virtual impedance different and after 0.5 second the difference goes almost zero even after 0.8 second sudden increase in load but voltage drop due to this action
- 19. 19/12 Advantages and disadvantages Advantages:- 1. Circulating current decrease aprrox. to zero by using this method. Disadvantages:- 1.Voltage deviations caused due to load variation is the major disadvantages of this method. 2. Secondary control and territory control also required for further control the grid system network
- 20. 20/12 Conclusion • Output impedance matching is one of the most important concept in converter • If impedance don’t match the circulating current will be flow in between the converter that are connected parallel • If the output resistance difference between the two converter is more, then the current sharing error and voltage regulation is deteriorated. (Fig.7)
- 21. 21/12 Conclusion 1.With higher value of virtual output impedance the load sharing has improved 2. Current sharing error between the converter will be reduced 3.Voltage deviation or droop will be more 4. For a smaller value of virtual impedance voltage deviation is less but current sharing error is higher. 5. Additional control i.e. secondary control for synchronization and territory control is also required to control the bidirectional power flow.
- 22. 22/12 Questions Arises 1. if there are more then two converter connected in parallel than this technique will be effective or not? i.) The value of virtual impedance changed every time or predefined. ii) If the value is predefine or fixed then how can we give the proportional feedback as we required to compensate the current sharing error. 2. What about the Cable resistance, It will be same or variable because all converter not at same distance. It could effect stability. 3. What about the output impedance- I think output impedance also a control variable in this case.
- 23. 23/12 4. Line impedance is hard to calculate because its may very with different load and in this approach conductor resistance also mentioned , so this is the point for confusion. 5. If circulating current is more and so after applied virtual impedance, voltage should not be more or less than maximum voltage deviation for example if output is 100 V and voltage deviation is +/- 5% so maximum voltage deviation will be 5 V so product of Virtual impedance and circulating current is less than 5 V, if more current not be removed.
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